Retention of Escherichia coli, Giardia lamblia cysts and Ascaris lumbricoides eggs in agricultural soils irrigated by untreated wastewater

In central Mexico, agricultural irrigation reusing Mexico City's municipal wastewater has been occurring for the last century, resulting in the recharge of the local aquifer. However, groundwater of this zone is of good quality, indicating that the microorganisms contained in wastewater are retained by soil after infiltration. This study aims to assess the capacity of three agricultural soils to retain three microorganisms frequently found in wastewater, namely Escherichia coli (E. coli), Giardia lamblia (G. lamblia) cysts and Ascaris lumbricoides (A. lumbricoides) eggs, through batch sorption-desorption assays. The tested soils were: an organic-clayey soil (C-OM), a clayey soil (C-om) and a sandy soil (c-om). For the three soils, sorption equilibrium of E. coli was reached before 1 h, while for G. lamblia cysts and A. lumbricoides eggs, sorption equilibrium took 2.5 h. Sorption of E. coli was better described by the Freundlich model than by the Langmuir one. Higher retention of bacteria was observed in the C-om soil (KF = 4340) than in the C-OM and c-om ones (KF = 1821 and 0.01, respectively). Regarding G. lamblia cysts and A. lumbricoides eggs, data could not be fitted to the tested sorption models. For both organisms, retention was lower in the C-OM soil than in the C-om and c-om ones. In the desorption tests, a sudden liberation of E. coli from soils was observed, probably due to bacterial re-growth. Desorption of G. lamblia was higher in the sandy soil than in the clayey ones; desorption was not increased when a surfactant was applied to the soil, suggesting that hydrophobic interactions are not necessarily responsible for retention of the cysts onto the tested soils. For A. lumbricoides eggs, desorption using NaOCl solution suggested that retention was caused by interactions between the mineral fraction of the soil and the external walls of eggs. This study showed that the three target microorganisms are retained by the tested soils and that mineral domain of soil has an important role in such retention.